I am definitely having this issue and it is getting pretty annoying. Yesterday the phone said it had a signal, but I didn't receive any calls or texts until I ran into my friend who had told me he called earlier and the phone went straight to voicemail. On my phone, it showed having full 3G signal.

Also, I was trying to send out an email and chat on gtalk but I kept getting the error even by switching the phone between offline and normal mode. This was indoors with a T-Mobile 3G connection.

The phone likes to switch between 2.5G and 3G signal as well even if I am in an area with a good 3G signal. I am not sure if the phone does it because it is trying to save power or if it is because it can't maintain a 3G signal.